(1) Field of the Invention
The present invention relates to novel electrochemical cells having electrolyte compositions containing specified compounds. More specifically, the present invention is directed to rechargeable, high energy density electrochemical cells having alkali metal anodes, chalcogenide cathodes and containing electrolyte compositions consisting essentially of solvent and electrolytically active alkali metal salts including a polyaryl metallic alkali metal salt.
(2) Prior Art
A recently developed rechargeable, high energy density electrochemical cell consists of an alkali metal material as the anode-active material, a transition metal chalcogenide as the cathode-active material, and a non-aqueous electrolyte. More specifically, preferred cells consist of lithium anodes, titanium disulfide cathodes and nonaqueous electrolyte compositions consisting of various lithium salts, such as LiClO.sub.4, dissolved in organic solvents, such as propylene carbonate, tetrahydrofuran, dioxolane, and mixtures of dimethyoxyethane and tetrahydrofuran, and containing various stabilizing additives.
Important features of these cells include their ability to be repeatedly discharged and charged. Theoretically, cycling by discharging and charging should be possible indefinitely, but in practice indefinite cycling is not realized. Dendritic growth on the anode during charging and degradation of the cathode material are sometimes limiting factors in the amount of cycling to which a cell can be subjected. However, the electrolyte, particularly nonaqueous electrolytes, can at times be the limiting factor. The effects of a particular electrolyte composition on the electrochemical performance of a cell may be significant due to its relative stability or it may be due to other factors. One particular electrolyte composition might be highly effective with a given anode-cathode couple but be ineffective for another couple, either because it is not inert to the second couple or because it reacts with itself under the conditions present during cycling. Furthermore, even when a particular electrolyte composition is effective in a given cell, it may nontheless be undesirable for other reasons. For example, the sometimes preferred LiClO.sub.4 based electrolyte creates a potential explosion hazard. And, for example, various organometallic alkali metal salt compounds such as are described in U.S. Pat. Nos. 3,734,963 and 3,764,385 have the disadvantage of requiring complexing with various nitrogen, phosphorus or sulfur-containing organic compounds containing at least two functionalities.
A study has been made directed to LiB(C.sub.6 H.sub.5).sub.4 electrolyte systems by Bhattacharyya, Lee, Smid and Swarc, J. Phys. Chem., Vol. 69, p. 608 et seq. (1965) but no suggestion is made therein that such systems may be used in cells containing alkali metal anodes. Also, the Bhattacharyya et al systems have been found to have low solubility and high resistivity. U.S. Pat. No. 3,935,025 describes anolytes and catholytes for sodium-containing batteries which contain specified alkali metal salts, e.g. NaB(C.sub.6 H.sub.5).sub.4, in organic solvents, but the reference fails to suggest the use of such systems having alkali metal anodes in combinAtion with chalcogenide cathodes. Copending U.S. patent application Ser. No. 750,517, now U.S. Pat. No. 4,060,674 entitled "Alkali Metal Anode-Containing Cells Having Electrolytes of Organometallic Alkali Metal Salts and Organic Solvents", filed on Dec. 14, 1976 by the present inventors now U.S. Pat. No. 4,060,674, described various organometallic alkali metal salt electrolytes, e.g. LiB(CH.sub.3).sub.4 and LiB(C.sub.6 H.sub.5).sub.3 CH.sub.3, and cells containing these, the salts being limited to those wherein at least one organic substituent is an alkyl radical. It has now been unexpectedly discovered that the salts used as electrolytes in the present invention having all aryl radicals as substituents exhibit superior gassing inhibition and have been found to be exceptional electrolytes for alkali metal anode/chalcogenide cathode cells in which gassing would otherwise be a problem. In fact, some of the preferred electrolytes of the above-mentioned copending application appeared to exhibit gassing which is typical of such electrolytes, whereas at least some of the electrolytes used in the present invention surprisingly appear to exhibit substantially negligible gassing.